{"id":602,"date":"2022-05-03T13:34:00","date_gmt":"2022-05-03T13:34:00","guid":{"rendered":"https:\/\/andrejacobs.org\/?p=602"},"modified":"2022-05-27T15:56:01","modified_gmt":"2022-05-27T15:56:01","slug":"learning-go-day-10","status":"publish","type":"post","link":"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/","title":{"rendered":"Learning Go &#8211; Day 10"},"content":{"rendered":"\n<p class=\"has-small-font-size\">Featured image by&nbsp;<a href=\"https:\/\/github.com\/egonelbre\/gophers\" target=\"_blank\" rel=\"noreferrer noopener\">Egon Elbre<\/a><\/p>\n\n\n\n<div class=\"wp-block-jetpack-markdown\"><h3>Custom Errors<\/h3>\n<ul>\n<li>You can export custom errors in your package and use the <code>errors<\/code> package to create them.<\/li>\n<\/ul>\n<pre><code class=\"language-go\">import &quot;errors&quot;\n\nvar (\n\tErrNotANumber = errors.New(&quot;Classic NaN issue&quot;)\n\tErrFailedToOpen = errors.New(&quot;Door is shut&quot;)\n)\n\/\/ Convention is to prefix with Err (or err if private)\n<\/code><\/pre>\n<ul>\n<li>Convention is to use <code>err<\/code> for the variable name receiving a potentially returned error.<\/li>\n<li>You can use <code>fmt.Fprintln<\/code> to output an error to <code>STDERR<\/code>.<\/li>\n<\/ul>\n<pre><code class=\"language-go\">fmt.Fprintln(os.Stderr, err)\n<\/code><\/pre>\n<ul>\n<li>Use <code>fmt.Errorf<\/code> to create new formatted errors which may also contain an existing error to be \u201cunwrapped\u201d with the <code>%w<\/code> specifier..<\/li>\n<\/ul>\n<pre><code class=\"language-go\">func something() error {\n\treturn fmt.Errorf(&quot;Encountered error %w because of %d&quot;, ErrFailedToOpen, 42)\n}\n\nerr := something()\nif err != nil {\n\tfmt.Fprintln(os.Stderr, err)\n}\n<\/code><\/pre>\n<h3>CSV file support<\/h3>\n<ul>\n<li>Go has a <a href=\"https:\/\/pkg.go.dev\/encoding\/csv@go1.18.1\">standard library package<\/a> for working with CSV files.<\/li>\n<li>Reading from a CSV file.<\/li>\n<\/ul>\n<pre><code class=\"language-go\">import &quot;encoding\/csv&quot;\n\nf, err := os.Open(fname)\n\/\/ handle err and defer closing of file\n\ncsvReader := csv.NewReader(f) \/\/ takes an io.Reader\n\/\/ Read all records into memory as [][]string\ndata, err := csvReader.ReadAll()\n\/\/ there is also Read to read a single row at a time\n<\/code><\/pre>\n<ul>\n<li>Writing to an io.Writer.<\/li>\n<\/ul>\n<pre><code class=\"language-go\">writer := csv.NewWriter(os.Stdout)\nwriter.WriteAll(data) \/\/ data = [][]string\n\n\/\/ or\nwriter.Write(row) \/\/ row = []string\n<\/code><\/pre>\n<h3>Benchmarking<\/h3>\n<ul>\n<li>On Unix style shells you can use the <code>time<\/code> command to time how long an operation took.\nNOTE: However the format of the Zsh built-in is different to the output of Bash. This can be changed by specifying the TIMEFMT environment variable. See <a href=\"https:\/\/unix.stackexchange.com\/questions\/562650\/time-command-is-different-in-zsh-than-in-bash\">this SO<\/a> post.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ TIMEFMT=$'real\\t%E\\nuser\\t%U\\nsys\\t%S'\n$ time sleep 2\nreal    2.01s\nuser    0.00s\nsys     0.00s\n<\/code><\/pre>\n<ul>\n<li>In Go you can provide benchmarking functions as part of the unit-tests.<\/li>\n<\/ul>\n<pre><code class=\"language-go\">func someBenchmark(b *testing.B) {\n\/\/ prepare some inputs etc.\n\n\tb.ResetTimer() \/\/ This ensure the setup time above is not part of the timings\n\n\tfor i := 0; i &lt; b.N; i++ {\n\t\t\/\/ code to be benchmarked\n\t}\n}\n<\/code><\/pre>\n<ul>\n<li>To run benchmarks you use <code>go test<\/code> and with the <code>-bench regexp<\/code> argument. It is good to ensure no unit-tests are run by using <code>-run ^$<\/code> (so that no unit-test match the regexp).<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -run ^$\n<\/code><\/pre>\n<ul>\n<li>b.N is adjusted so that the benchmark runs for roughly about 1 second. You can use <code>-benchtime=<\/code> argument to adjust this.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -benchtime=20x -run ^$ | tee results.txt\n<\/code><\/pre>\n<h3>Profiling<\/h3>\n<ul>\n<li>Go comes with built-in profiling and tracing tools.<\/li>\n<li>To perform a CPU profile while running a bench mark.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -benchtime=10x -run ^$ -cpuprofile cpu00.pprof\n\n$ go tool pprof cpu00.pprof\nType: cpu\nTime: May 3, 2022 at 7:56pm (BST)\nDuration: 3.63s, Total samples = 3.67s (101.09%)\nEntering interactive mode (type &quot;help&quot; for commands, &quot;o&quot; for options)\n(pprof)\n\n# enter 'top' to see where most of the time is spend\n(pprof) top\nShowing nodes accounting for 3.59s, 97.82% of 3.67s total\nDropped 29 nodes (cum &lt;= 0.02s)\nShowing top 10 nodes out of 82\n      flat  flat%   sum%        cum   cum%\n     2.99s 81.47% 81.47%      2.99s 81.47%  syscall.syscall\n     0.21s  5.72% 87.19%      0.21s  5.72%  runtime.madvise\n     0.12s  3.27% 90.46%      0.12s  3.27%  runtime.pthread_cond_wait\n...\n\n# Use 'top -cum' to see cummalitive time being spent\n(pprof) top -cum\n\n# Use 'list someFunction' to see source code listing and time spent on which lines.\n(pprof) list myPackage.thisTakesTooLong\n<\/code><\/pre>\n<ul>\n<li>To perform a memory profile.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -benchtime=10x -run ^$ -memprofile mem00.pprof\n\n$ go tool pprof -alloc_space mem00.pprof\n(pprof) top -cum\n...\n2.02GB 40.36% 75.11%     3.21GB 64.18%  encoding\/csv.(*Reader).ReadAll\n<\/code><\/pre>\n<ul>\n<li>To run a benchmark and also get a summary of allocations being made.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -benchtime=10x -run ^$ -benchmem | tee benchresults00m.txt\n...\nsomeBenchmark-8                10         311644288 ns\/op        495584000 B\/op   5041044 allocs\/op\n<\/code><\/pre>\n<ul>\n<li>pprof can generate a nice call \/ usage graph when you enter <code>web<\/code> at the pprof prompt. However you will need to have graphviz installed. (<code>brew install graphviz<\/code>).<\/li>\n<li>To compare two benchmarks you can use the \u201c<a href=\"http:\/\/golang.org\/x\/perf\/cmd\/benchstat\">benchstat<\/a>\u201d tool.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\"># To install it\n$ go install golang.org\/x\/perf\/cmd\/benchstat@latest\n\n# To compare two benchmarks\n$ ~\/go\/bin\/benchstat results00.txt results01.txt\n<\/code><\/pre>\n<h3>Tracing<\/h3>\n<ul>\n<li>This is awesome! Go provides a tool for tracing where a program spends its time, which CPUs are used etc. It uses your default browser to visualise and interact with.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">$ go test -bench . -benchtime=10x -run ^$ -trace trace01.out\n\n$ go tool trace trace01.out\n\n2022\/05\/03 20:35:40 Parsing trace...\n2022\/05\/03 20:35:41 Splitting trace...\n2022\/05\/03 20:35:41 Opening browser. Trace viewer is listening on http:\/\/127.0.0.1:51549\n<\/code><\/pre>\n<p><img decoding=\"async\" src=\"https:\/\/andrejacobs.org\/wp-content\/uploads\/2022\/05\/gotrace.png\" alt=\"\"><\/p>\n<h3>Miscellaneous<\/h3>\n<ul>\n<li>See package <code>strconv<\/code> for function to help convert between strings and data types. E.g. strconv.ParseFloat().<\/li>\n<li><code>io.Discard<\/code> is an <code>io.Writer<\/code> that can be used for when you don\u2019t care about the write operations. For example instead of passing io.Stdout to a function you can let it discard the output while unit-testing.<\/li>\n<li><code>iotest.TimeoutReader<\/code> can be used to simulate a timeout error while unit-testing.<\/li>\n<li>You can make a channel of empty struct to signal when an event has occurred. This avoids extra memory allocation for the channel.<\/li>\n<\/ul>\n<pre><code class=\"language-bash\">isDone := make(chan struct{})\n\n\/\/ in some go routine\nclose(isDone)\n\n\/\/ listen for signal\nfor {\n\tswitch {\n\tcase &lt;- isDone:\n\t\/\/ received signal\n\t}\n}\n<\/code><\/pre>\n<ul>\n<li>To get the number of CPU cores available use <code>runtime.NumCPU()<\/code>.<\/li>\n<\/ul>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Featured image by&nbsp;Egon Elbre [&#8230;]<\/p>\n<p><a class=\"understrap-read-more-link\" href=\"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/\">Read more &rarr;<\/a><\/p>\n","protected":false},"author":2,"featured_media":593,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_mi_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[52],"tags":[53],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v19.13 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Learning Go - Day 10 - Andr\u00e9 Jacobs<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Learning Go - Day 10 - Andr\u00e9 Jacobs\" \/>\n<meta property=\"og:description\" content=\"Featured image by&nbsp;Egon Elbre [...]Read more &rarr;\" \/>\n<meta property=\"og:url\" content=\"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/\" \/>\n<meta property=\"og:site_name\" content=\"Andr\u00e9 Jacobs\" \/>\n<meta property=\"article:published_time\" content=\"2022-05-03T13:34:00+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2022-05-27T15:56:01+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/andrejacobs.org\/wp-content\/uploads\/2022\/05\/welding.png\" \/>\n\t<meta property=\"og:image:width\" content=\"670\" \/>\n\t<meta property=\"og:image:height\" content=\"758\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Andr\u00e9 Jacobs\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@andrejacobs81\" \/>\n<meta name=\"twitter:site\" content=\"@andrejacobs81\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Andr\u00e9 Jacobs\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"4 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/andrejacobs.org\/study-notes\/learning-go-day-10\/\"},\"author\":{\"name\":\"Andr\u00e9 Jacobs\",\"@id\":\"https:\/\/andrejacobs.org\/#\/schema\/person\/3d38360883015e883c80c2fb875c5a68\"},\"headline\":\"Learning Go &#8211; 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